1. Field of the Invention
This invention relates to the field of Inductive Output Tubes. More particularly, this invention relates to Inductive Output Tubes for use as amplifiers and oscillators having coaxial output circuits and therefore having an anode and a collector arranged radially about a central cathode.
2. The Background Art
A major limitation to the power output obtainable from a conventional power grid tube is the power that can be dissipated by the grids, screens and anodes of such conventional tubes. Too much power dissipated into a wire grid can cause premature failure of the tube. A. V. Haeff, et al.'s Inductive Output Tube (IOT), developed in the 1930s and described in U.S. Pat. No. 2,225,447, uses nonintercepting electrodes, such as apertures, rather than delicate wire grids by employing a magnetic field disposed coaxially with the electron beam. Power is removed from the bunched or density-modulated electron beam by passing the beam through a resonant cavity in which the kinetic energy of the electrons, previously accelerated to a high velocity, is converted to electromagnetic energy without the need to collect the electrons on the walls of the cavity.
Inductive output tubes are thus a special family of tubes similar to tetrodes. They differ from conventional gridded tetrodes mainly by the way the radio frequency (RF) output power is extracted from the modulated electron beam inside the tube. While in the conventional tetrode both the screen grid and the anode form parts of the RF output circuit, the IOT features an output cavity separated from any beam current gating or collecting electrodes. The electron beam in the IOT interacts with the output cavity solely via electromagnetic field components, as in a klystron. Thus the amplitude of the RF output voltage is no longer limited to the DC potential difference between anode and screen grid, eliminating the typical tetrode compromise between gain and output power. As a result the IOT becomes an amplifier tube superior to the tetrode especially at UHF frequencies (300-3000 MHz), providing higher gain, efficiency and output power in this frequency range.
FIG. 1 is a schematic diagram of an IOT 10 according to the prior art. Electrons 12 from a thermionic cathode 14 are emitted and controlled by a grid 16 closely spaced from the emitting surface of cathode 14. Grid 16 is biased with a DC grid bias relative to cathode 14 as shown. A magnetic field 18 surrounds the linear electron beam 12. An RF signal (RF IN) to be amplified is introduced through input port 20 to input cavity 22. Interaction between the RF input signal in input cavity 22 and the electron beam 12 results in density modulation of the electron beam 12. Electrons are accelerated by a relatively high voltage on anode 25. In output cavity 26 between the anode 25 and the tailpipe as shown the density modulated current induces an electromagnetic field resulting in output power available through output coupling 28 of output port 30. The electrons are ultimately collected by a collector in a conventional manner.
Accordingly, the IOT has been perceived as a linear electron beam tube. IOTs built to date are consequently all of the linear beam type, using electron guns, output cavities and collectors similar to those of klystrons. This linear structure creates certain disadvantages. The output cavities for such a linear beam design employ preferably the TE.sub.101 mode (if rectangular) or the TM.sub.011 mode (if circular), as in klystrons. This leads to fairly bulky amplifier assemblies, which become especially awkward in the case of IOT-equipped television transmitters, where two coupled output cavities are normally required in order to achieve the specified bandwidth (approximately 6 MHz). An IOT designed to operate in coaxial output cavities (like those commonly used for tetrodes operating in the same UHF frequency spectrum) would lead to an amplifier with a considerably smaller footprint, thereby reducing equipment and site costs.
Another disadvantage linked with prior art IOTs is that in order to limit the space charge in the electron beam to values which still support a reasonable efficiency, and to extract output power at the desired levels despite limited availability of effective cathode surface area, the operating voltage of linear beam IOTs has to be even higher than that of klystrons of similar output power. Such IOTs typically operate in the Television Service at a voltage potential of about 30 to 38 KV for a power output in the range of about 40 to 75 KW. This high voltage (H.V. also denoted "+" and "-" in FIG. 1 as shown) requirement results in increased equipment costs for power supplies due to a consequent requirement for higher voltage insulation and more X-ray shielding. Additional adverse effects of such high voltage operation include the difficulty in preventing high-voltage arcing across the DC insulation that is an integral part of the input circuit in IOTs and an increased danger of high voltage breakdown in the cavity due in part to the fact that the peak RF voltage in the output circuit is higher than the operating voltage of the tube, all of which limit both the useable output power of the tube and the physical elevation above sea level at which the tube can be operated (due to reduced air pressure and breakdown of air dielectrics at altitude), if external cavities are used as they are for television transmission.
Current commercial television operators seek increased power output capabilities for television transmitters operating in the UHF frequency spectrum. Such transmitters are often operated on mountain tops and other high altitude locations having reduced air pressure and air dielectric breakdown voltages. Because power, P, voltage, V and current, I are related by the expression P=VI, more power can be obtained by operating a linear beam IOT at high voltage. However, as noted above, this apparently simple expedient, when implemented in reduced air pressure environments, requires substantial additional expense in power supplies, insulation, and the like, and is, as a practical matter, difficult and expensive to do. Similarly, more power can be obtained by increasing the electron beam current of the IOT, however, this is also difficult to achieve with current linear beam devices due to the space charge problems discussed above.
Accordingly, there is a need for a higher power UHF electron device which can achieve such higher output power with higher currents rather than by resorting to increased voltage operation.